1. Field of the Invention
The present invention relates to a track access control system in rotating disc devices. The present invention relates, in particular, to a track access and servo control system in optical magneto-optical disc devices.
Optical discs and magneto-optical discs are used as a large capacity external memories for their large data recording density. The optical discs and magneto-optical discs have narrow (typically 1.6 .mu.m width) tracks which are preformed spirally or concentrically on their surface, and each bit of data is recorded on a portion of a track.
In particular, in the magneto-optical discs, data is recorded as a state of magnetization in each portion of the tracks, and therefore new data can be written over a portion of a disc where the previously written data is recorded thereon. There are two types of magneto-optical discs depending on the types of the magnetic medium, one is overwritable and the other is non-overwritable. In the non-overwritable magneto-optical discs, an erasing operation is necessary before each writing operation.
Each bit of data is written on a portion of a track surface by impinging a high-intensity optical (light) beam onto the track surface for changing or not changing a magnetization state of the portion, and is read by impinging a low-intensity optical beam onto the track surface, and then detecting the polarization of an optical beam reflected at the track surface, where the polarization is caused by the magnetization state of the portion of the track at which the optical beam is reflected.
Generally, to carry out effective and accurate writing and reading operations, the optical beam impinged onto the track surface is required to exactly follow the track surface. To exactly follow the track, a track servo system is provided. Further, generally, when a data writing or reading command is received, the beam position is moved to the object track position, i.e., a track seek operation is carried out. In the seek operation, a velocity in moving the beam position and the time for moving the beam position are calculated, and a current for driving a motor to move the beam position, is given to the motor so that the motor is moved in the calculated velocity for the calculated duration.
However, in the current manufacturing technique of optical discs and magneto-optical discs, eccentricity of the tracks is inevitable; typically, the radius of a track varies in the range of about 50 .mu.m. Since the range of the variation of the radius is much larger than the track width, the eccentricity degrades the accuracy of the track jump operation, i.e., the eccentricity may lead to a track which is different from the target track depending on the phase in the rotation of the disc in the jump operation. The mislead in the track jump operation makes the operation unstable, and delays the track seek operation, and thus increases a total access time.
Further, the eccentricity of the tracks causes an instability in a track servo operation for maintaining the beam spot at an optimum position of an objective track on an optical disc or a magneto-optical disc.
2 Description of the Related Art
Nikkei Mechanical 1987, 7/13, pp. 73 (in Japanese) proposed a technique to correct the above mislead in the seek operation. In the proposed technique, first, the variation of the position of the track is measured by the optical position sensor, and is approximated as a sinusoidal waveform and is tabulated in a memory. In a track access operation, the deviation of the position of the track is read from the memory corresponding to the rotation angle (phase), is transformed to an analog signal, and is added to the position command for controlling the voice coil motor which moves an optical head.
However, in the above technique, the operation for memorizing a position information of the track in the approximated sinusoidal form, imposes a heavy load on the processor, and, in practice, the deformation of tracks cannot be necessarily approximated by a sinusoidal form.
U.S. application, Ser. No. 299,018 (filed on Jan. 19, 1989) discloses a track servo control system, wherein a variation of a track position signal due to the eccentricity of a track is detected and memorized in a memory. The memorized variation data is added to a control signal to drive a track actuator in the track servo loop. However, this technique does not provide a means to solve the above mislead in the track seek operation.